As an emission type electronic displaying device, there is an electroluminescence device (ELD). As elements constituting the ELD, there is an inorganic electroluminescent element or an organic electroluminescent element (hereinafter referred to also as organic EL element).
The inorganic electroluminescent element has been used for a plane-shaped light source, but a high voltage alternating current has been required to drive the element.
An organic EL element has a structure in which a light emission layer containing a light emission compound is arranged between a cathode and an anode, and an electron and a hole were injected into the light emission layer and recombined to form an exciton. The element emits light, utilizing light (fluorescent light or phosphorescent light) generated by inactivation of the exciton, and the element can emit light by applying a relatively low voltage of from several volts to several decade volts. The element has a wide viewing angle and a high visuality since the element is of self light emission type. Further, the element is a thin, complete solid element, and therefore, the element is noted from the viewpoint of space saving and portability.
An organic EL element for practical use is required which efficiently emits light with high luminance at a lower power. For example, there are disclosed an element with long lifetime emitting light with high luminance in which stilbene derivatives, distyrylarylene derivatives or tristyrylarylene derivatives are doped with a slight amount of a fluorescent compound (see for example, Japanese Patent No. 3093796), an element which comprises an organic light emission layer containing an 8-hydroxyquinoline aluminum complex as a host compound doped with a slight amount of a fluorescent compound (see for example, Japanese Patent O.P.I. Publication No. 63-264692), and an element which comprises an organic light emission layer containing an 8-hydroxyquinoline aluminum complex as a host compound doped with a quinacridone type dye (see for example, Japanese Patent O.P.I. Publication No. 3-255190).
When light emitted through excited singlet state is used in the element disclosed in the above Patent documents, the upper limit of the external quantum efficiency (ηext) is considered to be at most 5%, as the generation ratio of singlet excited species to triplet excited species is 1:3, that is, the generation probability of excited species capable of emitting light is 25%, and further, external light emission efficiency is 20%.
Since an organic EL element, employing phosphorescence through the excited triplet, was reported by Prinston University (for example, see M. A. Baldo et al., Nature, 395, p. 151-154 (1998)), study on materials emitting phosphorescence at room temperature has been actively made (for example, see M. A. Baldo et al., Nature, 403, 17, p. 750-753 (2000) or U.S. Pat. No. 6,097,147).
As the upper limit of the internal quantum efficiency of the excited triplet is 100%, the light emission efficiency of the exited triplet is theoretically four times that of the excited singlet. Accordingly, light emission employing the excited triplet exhibits the same performance as a cold cathode tube, and can be applied to illumination.
For example, many kinds of heavy metal complexes such as iridium complexes has been synthesized and studied (for example, see S. Lamansky et al., J. Am. Chem. Soc., 123, 4304 (2001)).
An example employing tris(2-phenylpyridine)iridium as a dopant has been studied (for example, M. A. Baldo et al., Nature, 395, p. 151-154 (1998)).
Further, an example employing as a dopant L2Ir (acac) (in which L represents a bidentate ligand, and “acac represents acetyl acetone) such as (ppy)2Ir (acac) (for example, see M. E. Tompson et. al., The 10th International Workshop on Inorganic and Organic Electroluminescence (EL′ 00, Hamamatsu)), or employing as a dopant tris(2-p-tolylpyridine)iridium {Ir(ptpy)3}, tris(benzo-[h]-quinoline)iridium {Ir(bzq)3}, or Ir(bzq)2ClP (Bu)3 has been studied (for example, see Moon-Jae Youn. Og, Tetsuo Tsutsui et. al., The 10th International Workshop on Inorganic and Organic Electroluminescence (EL′ 00, Hamamatsu)).
A hole transporting material is used as a host of a phosphorescent compound in order to increase emission efficiency (for example, see Ikai et. al., The 10th International Workshop on Inorganic and Organic Electroluminescence (EL′ 00, Hamamatsu)).
Various kinds of electron transporting materials are used as a host of a phosphorescent compound, and further doped with a new iridium complex (for example, M. E. Tompson et. al., The 10th International Workshop on Inorganic and Organic Electroluminescence (EL′ 00, Hamamatsu)). High emission efficiency is obtained by incorporation of a hole blocking layer (for example, see Moon-Jae Youn. Og, Tetsuo Tsutsui et. al., The 10th International Workshop on Inorganic and Organic Electroluminescence (EL′ 00, Hamamatsu)).
At present, an organic electroluminescent element emitting phosphorescence with further higher emission efficiency and longer lifetime has been studied.
An external qauntum efficiency of around 20%, which is a theoretical threshold, is attained in green light emission, but in a low current region (a low luminance region), and the theoretical threshold is not attained in a high current region (a high luminance region). Further, a sufficient emission efficiency is not attained in another color emission, where there is room to be improved. An organic EL element for practical use is required which efficiently emits light with high luminance at a lower power. Particularly, an organic EL element is required which emits a blue phosphorescence with high efficiency.
As a hole transporting material used in a conventional organic EL element employing phosphorescence emission, α-NPD, m-MTDATA, TPD, hm-TDP, or PEDOT or PVK as a polymer type is used.
α-NPD, which is most generally used, easily injects holes to a light emission layer, however, its performance as a hole transporting material of an organic EL element emitting a green phosphorescence is not sufficient, since its exited triplet energy is low. Accordingly, its performance as an organic EL element emitting a blue phosphorescence is not satisfactory.
m-MTDATA easily injects holes to a light emission layer, and its exited triplet energy is relatively high, however, its performance as an organic EL element emitting a blue phosphorescene is not satisfactory.
TDP, hm-TDP (literature) is not suitable for an organic EL element emitting a blue phosphorescence, and has problem in view of lifetime.
The exited triplet energy of PEDOT is extremely low, and PEDOT does not have sufficient performance as a hole transporting material of an organic EL element emitting phosphorescene.
PVK is excellent in view of extremely high exited triplet energy of PEDOT, however, it has problem in view of transportability of holes, since its ionization potential is very high.

Even when the above compounds are used, layer constitution can be designed so that triplet excitons are not inactivated by a hole transporting layer, however, such a design is very difficult, and almost impossible particularly in a high current region (high luminance region).